1. Field of the Invention
The present invention relates to a visual inspection method of and a visual inspection apparatus for performing a visual inspection on an inspection target sample by using an imaging process. More particularly, the present invention relates to a technique for minutely inspecting a shape of a protrusion of a BGA (Ball Grid Array).
2. Description of the Related Art
Conventionally, a visual inspection apparatus and a visual inspection method have been known for taking a photograph of an integrated circuit (IC) having a package of a BGA type, and then inspecting an appearance of the integrated circuit by using a picture obtained by this photograph operation.
As such a visual inspection apparatus, for example, Japanese Laid Open Patent Application (JP-A-Heisei 9-311014) discloses “Apparatus for Inspecting Protrusion of Semiconductor Integrated Circuit Apparatus”. This apparatus for inspecting a protrusion compares a rough coordinate of a protrusion targeted for an inspection with a coordinate at which the protrusion should be originally existed. As this comparison result, if it is so judged that a position of the protrusion is largely deviated from a position defined by an inspection standard, a position deviation error signal is outputted. Then, the inspection of the protrusion is stopped, and the inspection of a next protrusion is executed. On the other hand, if it is judged that the protrusion is not largely deviated from the position defined by the inspection standard, a high accurate judgment of the position is requested.
In response to this request of the high accurate judgment of the position, a sub-pixel protrusion coordinate is compared with the coordinate at which the protrusion should be originally existed. Then, it is judged that a positional deviation error is occurred if the comparison result is not within a preset allowable range of a positional deviation. The above-mentioned process enables a positional deviation of a solder protrusion to be quickly inspected at a high accuracy.
Japanese Laid Open Patent Application (JP-A-Heisei 8-203972) discloses “Protrusion Inspecting Apparatus”. In this protrusion inspecting apparatus, an inspection target surface is illuminated from a horizontal direction or an obliquely upward direction close to a horizontal direction, and a photograph of the inspection target surface is taken by using a camera placed above the inspection target surface. Then, a binary conversion process is performed on shade picture data obtained from the photograph operation, and a labeling process is performed on the binary converted shade picture data. As for a judgment of a pass or a rejection, in a first method, the number of labels having a region of a normal size is compared with the predetermined number of protrusions, and it is judged as the pass if they coincide with each other.
In a second method, a central coordinate of a label corresponding to a protrusion detected by the inspection in the last time is used as a start point. Then, a label of a next protrusion having a central coordinate at a coordinate separated by a predetermined distance from this start point is sequentially retrieved, and it is judged as the pass if labels corresponding to all protrusions are retrieved. Thus, it is possible to detect the defects such as a breaking and a size error of a protrusion formed by a solder ball on a BGA board, a bump on a semiconductor apparatus and the like.
As another related art, Japanese Laid Open Patent Application (JP-A 2000-65543) discloses “Bump Illuminating Method and Apparatus, Bump Photographing Method and Apparatus, Picture Processing Method and Apparatus, Bump Inspecting Method and Apparatus, And Information Storage Medium”. In this technique, a solder bump is illuminated from all circumference directions, and a photograph of the solder bump is taken while a light amount at a center is reduced. The quality of a coating state of reinforcement resin on the solder bump is automatically inspected by extracting an object from the picture data obtained by the photograph operation, and then confirming an area and an aspect ratio. Thus, it is possible to easily inspect the quality of the spherical solder bump on which lower half the reinforcement resin is coated after the solder bump is mounted on a surface of a circuit board.
As still another related art, Japanese Laid Open Patent Application (JP-A 2000-121338) discloses “Apparatus for Inspecting Electronic Part”. In this apparatus for inspecting the electronic part, a cylindrical illuminating unit is placed above a lens of a camera. Two ring-shaped light source rooms, each of which is constituted by three interiorly projected flanges, are formed in upper and lower two stages, in an upper opening portion of the illuminating unit. A light source composed of a plurality of LEDs is placed at each of their tips so that a ring-shaped illuminator is formed. A part retainer, such as a work head and the like, for a part loading apparatus, which is located close to the upper opening portion, retains a BGA electronic part at a tip of an adsorption nozzle, and waits for the photograph operation. The photograph of a bump having a normal shape taken by the camera is obtained as a ring-shaped optical picture having a uniform ring width and being continued in a dark background. The pass or rejection is judged by measuring a diameter of the optical picture and the width of the ring from an angle at which the optical image is divided into at least eight portions and then comparing with a preset data. According to this apparatus for inspecting the electronic part, it is possible to judge the pass or rejection of the bump of the BGA electronic part surely and quickly.
However, in each of the techniques disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 9-311014) and Japanese Laid Open Patent Application (JP-A-Heisei 8-203972), the shade picture data captured by using the camera is converted into the binary numeral in a pixel unit. Then, the pass or rejection of the protrusion is judged based on the binary picture data obtained by the above conversion. Thus, the inspection accuracy depends on whether or not there are a large number of pixels in the binary picture data. Hence, there may be a case that a visual inspection apparatus using a cheap element for taking a photograph can not detect the shape defect. This results in a problem that the inspection accuracy is low.